Bipolar disorder (BPD) is a brain disorder characterized by recurrent manic and major depressive episodes with a one year prevalence rate between 1-2%.1 BPD ranked 20th in terms of causes of loss of disability- adjusted life-years in 19992 and is associated with a life time suicide risk of up to a 19%.3 The main burden of illness in BPD is in the depressive pole. A deficiency of serotonin (5-HT) function has been postulated to underlie depressive episodes yet few studies have examined indices of 5-HT neurotransmission in the brain in BPD. It is widely acknowledged that there are significant gaps in the current identification and treatment of bipolar depression.5-8 Better understanding of the neurotransmission deficits in BPD may aid diagnosis, identification of biomarkers and treatment targets to facilitate treatment development and ultimately to assist in treatment selection. Our preliminary data with [11C]DASB shows lower binding in BPD. We propose to determine the extent and nature of abnormalities of 5-HTT binding in vivo using positron emission tomography (PET) in medication-free bipolar I depression. We hypothesize that BPD has lower 5-HTT binding compared to controls. We will also investigate the 5-HT effects of a common treatment for BPD, lithium. Discovered decades ago, lithium remains one of the few effective treatments in BPD, with evidence of mood stabilizing, antidepressant, antisuicidal, and even neuroprotective qualities and is considered to be first line treatment. The actions of lithium on 5-HT indices may be central to its antidepressive and antisuicidal properties. We hypothesize that lithium downregulates presynaptic 5-HT1A receptor binding, upregulates postsynaptic 5-HT1A binding, upregulates 5-HTT binding, and these molecular effects will be related to clinical improvement, both in depression and suicidality. 5-HT1A binding potential will be determined using [11C]WAY 100635. We propose to perform [11C]DASB and [11C]WAY 100635 scans in 38 medication free BPD I subjects during a major depressive episode and compare 5-HTT and 5-HT1A binding potential in 38 healthy volunteers. We will also examine the diagnostic specificity of lithium response by studying 10 unipolar depressed subjects in an identical manner. We will examine the pharmacological specificity of lithium by studying lamotrigine in BPD subjects. Finally, we will also assess the ability of baseline scanning to predict treatment response. All BPD subjects will be treated with lithium and have repeat scans with both radiotracers. Many with BPD do not tolerate lithium's side effect burden, and it has a narrow therapeutic window. In this grant period we will determine at which 5-HT protein(s) lithium exerts its antidepressant and antisuicidal properties. Ultimately this can lead to novel therapeutics that are better tolerated. We will independently advance our understanding of the molecular pathophysiology of BPD as well as characterize the mechanisms of action of lithium.